YP is the first period of China's Landscape Stamped Postcard, issued from 1984 to 1994. During the 11 years, 16 different sets of YP was issued. And each YP has A series (domestic postage) and B series (international postage).
The first issue is YP1 Landscapes of Guilin, issued in Aug 20, 1984. The last issue is YP16 Hubei Landscapes, issued in Nov 11, 1994.
After 1994, the FP was issued.